The amounts in which material is suspended in different kinds of aqueous suspensions is an important measuring parameter, not least from the aspect of environmental care and protection. By suspended material is meant here generally such substances as those which can be separated mechanically by filtration and centrifugation. Especially in the forest industries, the suspended material may consist of many different components such as fibers, fiber fragments and various fillers and coating agents. These components may vary widely in size, from a few millimeters in length and some tens of microns in width (fibers) down to a particle diameter of about 1 micron and smaller (filler). The concentration of a suspension can vary within wide limits, from some mg/l to tens of g/l. It is important not only to measure the content of suspended material, but in many cases also to indicate when the size distribution of the suspended material changes, and the extent of this change. One area of use in this respect is to indicate the effect of the addition of floculating chemicals.
The majority of instruments at present available on the market for continuously measuring suspended materials are based on optical measurement principles, e.g., on light absorption, light scattering and the influence of polarized light. The most common method comprises measuring the turbidity of the suspension, in which attenuation or scattering of light is used as a measurement of the suspended-material content. The extent to which light is scattered, however, depend not only on the concentration of the suspended material, but also on the particle size, shape, surface structure and refraction index of the material concerned.
Thus, suspensions in which the particle-size distribution varies considerably can give misleading information with regard to the concentration of the suspension. This is illustrated in FIG. 22, which discloses the result of turbidity measurements in which attenuation of the light transmitted through suspension was measured. The result (the output signal from the measuring apparatus) is shown as a function of the concentration of cellulose fibers (large particles) and for clay (small particles). As will be seen, such an instrument is far more sensitive to clay than to fibers for a given concentration.
U.S. Pat. No. 4,110,044 describes an optical method which, in principle, enables the concentration of suspended material to be determined independently of particle size. This measuring principle is based on recording not only the mean value of the light transmitted, but also on measuring fluctuations in light intensity, in the form of a signal which includes the square of the true effective value of an alternating voltage component of the measurement signal obtained. As a function of particle size, this value has a mirror-reversed behavior in relation to the signal which is based on the direct voltage component of the measurement signal. Consequently, the sum of these two signals will provide a measurement of the amount of material in the suspension independently of particle size.
Furthermore, it is possible to obtain a relative measurement of the particle-size distribution of the suspended material, by forming the quotient of these two signals.
This method is called the TP-method and is quite effective, particularly in the case of low concentrations, where good linearization of both the direct voltage signal and the square of the true effective value can be obtained.
However, in the case of suspensions having a particle-size distribution in which the particles are predominently large particles, the signal formed by the square of the effective value is influenced to a greater extent by the large particles than by the small particles. Another drawback, and one which is often serious in practice, is that the TP-method is relatively difficult to calibrate.
Another method which indicates the direct voltage component of the measuring signal and the effective value of the alternating voltage component of the measuring signal is described in U.S. Pat. No. 3,879,129. This has the same drawbacks as the TP-method.
In order for the TP-method, and similar methods based on measuring the effective value of the measuring signal to provide good accuracy with respect to particle content and good resolution with respect to particle size, it is necessary (depending on particle content and particle size) for the measured volume to be small and the light beam to be narrow. If essentially each particle is to produce a significant indication of the alternating voltage component of the signal, the illuminating light beam should be narrow and preferably collimated or focused, and the reflected light should be detected within a narrow angular range. The variations in alternating voltage are smoothed out when the light beam has a broad path. It is often desired to measure the suspended-material content of suspensions in which the proportion of suspended material is relatively high. In order to enable light radiated through the suspension to be indicated at all, the light source must be located comparatively close to the light detector. Consequently, it is usual to place the measuring apparatus in a narrowed or necked part of the measuring cell. This narrowed area of the measuring cell is prone to become blocked with large particles, suspended material-agglomerates and the like. Consequently, these narrowed measuring cell-areas are equipped with back-flushing devices and the like by means of which said area can be rinsed clean, when necessary.
One example of an arrangement of this kind is described in the article LASER OPTICAL METHOD FOR DYNAMIC FLOCCULATION TESTING IN FLOWING DISPERSIONS, by W. Ditter et al, BASF Aktiengesellschaft, from the book "The Effect of Polymers on Dispersion Properties", Academic Press, London 1982, pages 353-342. The suspension is led through a narrowed or necked measuring cell equipped with a measuring head arrangement. Light from a laser is focused onto the measuring cell and light exiting from the measuring cell is led to a photosensor. Another example of a similar arrangement having a short narrowed or necked part is described in U.S. Pat. No. 3,879,159 FIG. 2B. This arrangement also includes a known measuring head having an open channel which is intended to be immersed in a flowing suspension. This measuring head, which is based on fiber-optic techniques described in U.S. Pat. No. 3,879,159 has a very deep measuring gap or throat. Placed on the bottom of the gap is an optic which functions to emit light to the suspension flowing through the gap and to receive light exiting from the suspension. Because the gap is very deep, it will, in principle, function as a narrowed or necked measuring cell, and consequently will also give rise to the aforesaid blocking problem.